Trieste consisted of a float chamber filled with gasoline (petrol) for buoyancy, with a separate pressure sphere to hold the crew. This configuration (dubbed a bathyscaphe by the Piccards), allowed for a free dive, rather than the previous bathysphere designs in which a sphere was lowered to depth and raised again to the surface by a cable attached to a ship.

Trieste was designed by the Swiss scientist Auguste Piccard and originally built in Italy. His pressure sphere, composed of two sections, was built by the company Acciaierie Terni. The upper part was manufactured by the company Cantieri Riuniti dell'Adriatico, in the Free Territory of Trieste (on the border between Italy and Yugoslavia); hence the name chosen for the bathyscaphe. The installation of the pressure sphere was done in the Cantiere navale di Castellammare di Stabia, near Naples. Trieste was launched on 26 August 1953 into the Mediterranean Sea near the Isle of Capri. The design was based on previous experience with the bathyscaphe FNRS-2. Trieste was operated by the French Navy. After several years of operation in the Mediterranean Sea, the Trieste was purchased by the United States Navy in 1958 for $250,000.

At the time of Project Nekton, the Trieste was more than 15 m (50 ft) long. The majority of this was a series of floats filled with 85,000 litres (22,000 US gal) of gasoline, and water ballast tanks were included at either end of the vessel, as well as releasable iron ballast in two conical hoppers along the bottom, fore and aft of the crew sphere. The crew occupied the 2.16 m (7.09 ft) pressure sphere, attached to the underside of the float and accessed from the deck of the vessel by a vertical shaft that penetrated the float and continued down to the sphere hatch.

The pressure sphere provided just enough room for two people. It provided completely independent life support, with a closed-circuit rebreather system similar to that used in modern spacecraft and spacesuits: oxygen was provided from pressure cylinders, and carbon dioxide was scrubbed from breathing air by being passed through canisters of soda-lime. Power was provided by batteries.

Don Walsh and Jacques Piccard aboard Trieste

The Trieste was subsequently fitted with a new pressure sphere, manufactured by the Krupp Steel Works of Essen, Germany, in three finely-machined sections (an equatorial ring and two caps).

To withstand the enormous pressure of 1.25 metric tons per cm² (110 MPa) at the bottom of Challenger Deep, the sphere's walls were 12.7 centimetres (5.0 in) thick (it was overdesigned to withstand considerably more than the rated pressure). The sphere weighed 14.25 metric tons in air and eight metric tons in water (giving it an average specific gravity of 13/(13-8) = 2.6 times that of sea water). The float was necessary because of the sphere's density: it was not possible to design a sphere large enough to hold a person that could withstand the necessary pressures, yet also have metal walls thin enough for the sphere to be neutrally buoyant. Gasoline was chosen as the float fluid because it is less dense than water, incompressible even at extreme pressure, thus retaining its buoyant properties and negating the need for thick, heavy walls for the float chamber.

Close-up of pressure sphere, with forward ballast silo at left

Observation of the sea outside the craft was conducted directly by eye, via a single, very tapered, cone-shaped block of acrylic glass (Plexiglas), the only transparent substance identified which would withstand the external pressure. Outside illumination for the craft was provided by quartz arc-light bulbs, which proved to be able to withstand the over 1,000 atmospheres of pressure without any modification.

Nine tons of magnetic iron pellets were placed on the craft as ballast, both to speed the descent and allow ascent, since the extreme water pressures would not have permitted compressed air ballast-expulsion tanks to be used at great depths. This additional weight was held in place at the throats of two hopper-like ballast silos by electromagnets, so in case of an electrical failure the bathyscaphe would automatically rise to the surface.

Transported to the Naval Electronics Laboratory's facility in San Diego, California, Trieste was modified extensively by the Americans, and then used in a series of deep-submergence tests in the Pacific Ocean during the next few years, culminating in the dive to the bottom of the Challenger Deep during January 1960.

23 January 1960: Trieste just before the record dive. The destroyer escort USS Lewis is in the background.

Trieste departed San Diego on 5 October 1959 for Guam aboard the freighter Santa Maria to participate in Project Nekton, a series of very deep dives in the Mariana Trench.

On 23 January 1960, she reached the ocean floor in the Challenger Deep (the deepest southern part of the Mariana Trench), carrying Jacques Piccard and Don Walsh.[1] This was the first time a vessel, manned or unmanned, had reached the deepest known point of the Earth's oceans. The onboard systems indicated a depth of 11,521 metres (37,799 ft), although this was revised later to 10,916 metres (35,814 ft); fairly recently, more accurate measurements have found Challenger Deep to be between 10,911 metres (35,797 ft) and 10,994 metres (36,070 ft) deep.[2]

The descent to the ocean floor took 4 hours 47 minutes at a descent rate of 0.9 metres per second (3.0 ft/s).[3][4] After passing 9,000 m (30,000 ft) one of the outer Plexiglas window panes cracked, shaking the entire vessel.[5] The two men spent barely twenty minutes on the ocean floor, eating chocolate bars for sustenance[citation needed]. The temperature in the cabin was 7°C (45°F) at the time. While at maximum depth, Piccard and Walsh unexpectedly regained the ability to communicate with the support ship, USS Wandank (ATA-204), using a sonar/hydrophone voice communications system.[6] At a speed of almost 1.6 km/s (1 mi/s) – about five times the speed of sound in air – it took about seven seconds for a voice message to travel from the craft to the support ship and another seven seconds for answers to return.

While at the bottom, Piccard and Walsh observed a number of small sole and flounder.[7] Their claim the fish were swimming would prove at least some vertebrate life can withstand the extreme pressure at the oceans' deepest point.[8] They noted that the floor of the Challenger Deep consisted of "diatomaceous ooze". The ascent took 3 hours 15 minutes.

Beginning in April 1963, Trieste was modified and used in the Atlantic Ocean to search for the missing submarine USS Thresher (SSN-593). Trieste was delivered to Boston Harbor by USS Point Defiance (LSD-31) under the command of Captain H. H. Haisten. In August 1963, Trieste found the wreck off the coast of New England, 2,600 m (8,400 ft) below the surface.[9]Trieste was changed, improved and redesigned so many times that almost no original parts remain. She was transported to the Washington Navy Yard where she was exhibited along with the Krupp pressure sphere in the National Museum of the U.S. Navy at the Washington Navy Yard in 1980. Her original Terni pressure sphere was incorporated into the Trieste II.

The book "Ten Miles High, Two Miles Deep" (1957) (Whitlesey House, ISBN B0006AV2JW) documents Picard's research on the stratosphere, as well as his work with the Trieste, especially the design and construction.

The song 'The Trench' by Danish composer Ste van Holm is a tribute to the Mariana Trench dives.

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